1. Field of the Invention
The present invention relates to improvements in methods and apparatus for detecting the liquid level in one or more liquid-storage containers. The apparatus of the invention is particularly useful in automated laboratory instruments, e.g., hematology and other flow-cytometric instruments, that gradually consume liquids of different types and require that the respective levels of such liquids be maintained above a certain level in their storage containers for effective operation of the instrument.
2. The Prior Art
Many automated laboratory instruments that operate to analyze the content of test samples presented to them for analysis consume a variety of liquids in performing their analytical tasks. For example, in analyzing a multitude of different blood samples, hematology and other flow-cytometric instruments will gradually consume a host of different liquid reagents, e.g., lyse, lyse-stabilizer, stains, antibodies, etc., which are selectively mixed with small portions of each sample to condition such portions for subsequent analysis or testing. Such instruments also consume relatively large amounts of non-reactive liquids, such as diluents, that serve not only to reduce the concentration of the blood sample and/or reagents prior to analysis or mixing, but also to cleanse different blood-contacting components of the instrument system (e.g., aspiration and dispensing probes, mixing chambers, blood-sampling valves, etc.) after each sample analysis in order to prevent sample carry-over from one analysis to another. In addition to consuming various liquids, these instruments produce liquid waste products in the form of non-used portions of the reactive and non-reactive liquids, and blood-containing mixtures that have been used for analysis. As produced, these liquid waste products are commonly directed to one or more waste containers that must be emptied or replaced after the waste liquid reaches a certain fill level. As the reactive and non-reactive liquids are consumed, and as the liquid waste products are accumulated, it is necessary to frequently detect the level of each liquid in its storage container. Should any one of the liquids used for an analysis be allowed to “run dry” in the middle of a test, inaccurate results will be reported. Similarly, should a container of waste liquid overflow its storage container, a biologically hazardous clean-up process is necessitated. Thus, before each analysis is initiated, it is common in such instruments to check all of the liquid levels to assure that there is an ample supply of liquids needed for a particular test, and to assure that there is ample volume in the waste storage container(s) to accept all liquid waste products resulting from such test.
Reagents and diluents of the type noted above are commonly supplied by their respective manufacturers in relatively large containers of different sizes and shapes. Rather than providing a liquid-level-detector that is custom designed for each of the different supply containers, it is common to selectively pump or otherwise transfer a portion of the liquids from their respective supply containers (as provided by the manufacturer) to a relatively small “buffer” container located aboard the instrument. These buffer containers are usually of a standard size, e.g., 5 to 100 milliliters in volume, and are preferably shaped so that the same type of liquid level sensor can be used in each container to detect the liquid level therein. Ideally, the liquid-level-sensor should be capable of detecting the level of liquid within the buffer containers at different levels, e.g., at a “FULL” level, indicating that there is ample liquid to perform any sample analysis required of a given container, and at an “ALMOST EMPTY” level, indicating that the liquid level is dangerously close to a level below which air may enter the reagent-dispensing lines, thereby compromising the results of the analysis and/or requiring a time-consuming re-priming operation to restore the operation of the instrument. In response to a signal indicating an “ALMOST EMPTY” level has been reached, a pump or the like is usually activated to restore the liquid-level in the buffer container to the “FULL” level. In some cases, the liquid-level-sensor should also be adapted to sense an “EMPTY” level to advise the user that the level of liquid has dropped to a level at which start-up and priming procedures are required, and/or that the container is substantially void of liquid.
To detect multiple liquid-levels in buffer containers of the type noted above, it is common to use float-triggered liquid-level sensors located within the buffer container or directly within the much larger reagent supply container. Such float sensors typically comprise an elongated stem member that is vertically supported within the container, and a float member that is slidably-mounted on the stem. In use, the float rests atop the liquid in the container and, as the liquid level within the container rises and falls, the buoyant force of the liquid causes the float to slide vertically along the stem. Often, the float has an embedded magnetically-actuated electrical switch that is actuated by a plurality of axially-spaced magnetic members that are embedded within the stem. Thus, as the float moves up and down the stem in response to a change in liquid level, its embedded electrical switch will open or close as it passes each of the embedded magnetic members in the stem. Different liquid levels within the container can be detected by appropriately positioning the magnetic members in the stem at strategic locations. Typical float sensors of the type described are disclosed, for example, in U.S. Pat. No. 6,513,378 to Love, and U.S. Pat. No. 6,253,611 to Varga et al. A variation of this type of sensor is disclosed in U.S. Pat. No. 5,562,003 to Lefebvre wherein multiple magnetic switches are embedded in the float-guiding member and the float member carries a magnetic member that activates the switches as it floats in close proximity. While the float sensors of the prior art are readily capable of sensing multiple liquid levels in a container, they may be considered disadvantageous from the standpoints that (1) the float member must be relatively large in size in order to be buoyant, thus requiring the buffer containers to be correspondingly large in size to accommodate the float, (2) the container opening must be sufficiently large to enable installation of the float within the container, and (3) the float must be made of a material that does not chemically react with the different reagents and other liquids.
It is known in the art to sense the liquid level in a container by disposing the container on an electrically-conductive plate and lowering an electrically-grounded electrode, e.g., comprising a liquid-aspiration probe, into the container until it contacts the liquid therein. By biasing the plate with an alternating current, a change in capacitance between the electrode and plate is detected as the electrode tip moves from one dielectric medium to another, e.g., from air to liquid. While such a scheme is useful for detecting a single level of liquid, it is not useful for detecting multiple fixed levels of liquid in a container. Such a liquid level-sensing apparatus is described in the commonly assigned U.S. Pat. No. 4,326,851 to Bello et al.